In semiconductor device fabrication processes, in order to observe defects that cause device failures, automatic defect observation is performed wherein an image where a defect detected by an optical inspection device or electron beam-based inspection device is located is re-detected at a high resolution by means of a review SEM device as an application of a scanning electron microscope. Because of the reduction in size of semiconductor device circuit patterns and of the resultant reduction in defect size, such review SEM devices are required to be capable of highly reliable and high-resolution automatic defect observation for defects detected by an inspection device.
With respect to automatic defect observation by review SEM devices, by way of example, the following are known. A review SEM selects one defect from among specified defects, and, at a first magnification setting, images the same site of a chip adjacent to the chip where the defect site is located within the wafer. This site is a site at which the same pattern as that of the defect site is formed, and this image is referred to as a low-magnification reference image. Next, the stage is moved so as to place the defect position at the center of the optical system's view. Then, as when the reference image was obtained, an image is obtained at the first magnification setting. This image is referred to as a low-magnification defect image. Then, based on the defect image and reference image of the first magnification setting, the defect site is identified, and the defect site is imaged at a second magnification setting, which is greater than the first magnification setting for imaging. This image is referred to as a high-magnification defect image. Thus, with review SEM devices, a low-magnification image of a first magnification setting and a high-magnification image of a second magnification setting are obtained in stages.
With review SEM devices whose primary objective is to observe defects in detail based on high-magnification defect images, when obtaining a high-magnification defect image at the second magnification setting, automatic focal position alignment (hereinafter “autofocusing”) is performed, where a focusing position with respect to the observed region is automatically calculated. In particular, semiconductor wafers generally have variability in the height direction or are charged, and the focusing position varies depending on the observed region. In order to obtain a high-resolution image with a review SEM device, the focal position of the electron beam must be set to the focusing position each time through autofocusing. The term focusing position as used herein refers to a focal position where the focal point is set with respect to a sample surface, pattern defect, or foreign matter, and at which it is possible to obtain a high-resolution image.
By way of example, an autofocusing process is performed as follows. A plurality of images with varying focal positions are obtained based on predetermined autofocusing conditions. With respect to each of the thus obtained images, a focal point measure, which is an index quantifying the sharpness of the focal point, is calculated. The focal position at which the focal point measure becomes greatest is estimated and set. In general, the sharpness of an edge part captured within an image is often used for the focal point measure, and the value of the focal point measure also increases in accordance with the proximity to the focusing position.
With respect to autofocusing conditions, as a parameter for changing the balance between autofocus precision and the time required for autofocusing, there is the change in focal position in obtaining a plurality of images with varying focal positions (hereinafter “step size”).
With respect to step size for autofocusing, by making it finer, more images of varying focal positions are obtained, and errors in the estimated value of the focusing position decrease. However, depending on the defect being imaged, or the material or structure of the pattern, there may be cases where image contrast cannot be obtained, edge sharpness decreases, and a sufficient focal point measure cannot be obtained. In addition, when a large number of images are obtained by varying the focal position by excessively fine step sizes, the time required for autofocusing increases in proportion to the number of images obtained. Accordingly, the operator configures settings taking the balance between autofocus precision and the time required for autofocusing into consideration. Further, in order for the operator to determine the optimal setting for step size, he has to go through a repeated process of trial and error, and the automation of condition setting for the step size to be used for autofocusing is therefore desired.
As a known method of automatically determining autofocusing conditions, there is disclosed in JP Patent Application Publication (Kokai) No. 2009-194272 A (Patent Literature 1) a method in which autofocusing is performed at a plurality of pre-registered coordinate positions with respect to a coordinate system of an observed object, and a search range for autofocusing is automatically determined based on the focal position and focus offset amount at each point.
In addition, as a method of determining autofocusing conditions for high-magnification image acquisition using information obtained from a low-magnification image, there is presented in JP Patent Application Publication (Kokai) No. 2005-285746 A (Patent Literature 2) a method in which a region with an edge strength equal to or greater than a threshold is determined within a region of an obtained low-magnification image, a region containing more than a certain number of pixels of high edge strengths is set as a region for autofocusing, and autofocusing is executed.
In addition, there is presented in JP Patent Application Publication (Kokai) No. 10-050245 A (1998) (Patent Literature 3) an auto focusing method in which a maximum effective scan interval of an electron beam is set, this maximum effective scan interval is compared with the actual scan interval that is based on a magnification that has been set, and, if the actual scan interval is equal to or greater than the maximum effective scan interval, the magnification in the scanning direction of the electron beam is increased to lower the actual scan interval to or below the maximum effective scan interval.